As a fixing device used in an electrophotographic image forming apparatus such as a copying machine or a printer, a fixing device employing a heat roller fixing system is often used. The fixing device employing the heat roller fixing device includes a pair of rollers (a fixing roller and a pressure roller) that press against each other. Inside both or either one of this pair of rollers, heating means made of, for example, a halogen heater is provided. After the pair of rollers are heated to a predetermined temperature (target fixing temperature) by the heating means, recording paper on which unfixed toner image is formed is fed into a pressure area (fixing nip area) between the pair of rollers. When the recording paper passes through the pressure area, the unfixed toner image is fixed by heat and pressure.
Note that in a fixing device provided in a color image forming apparatus, it is common to use an elastic roller which has an elastic layer made of, for example, silicone rubber which elastic layer is provided on a surface layer of the fixing roller. Because the fixing roller is an elastic roller, a surface of the fixing layer is elastically deformed in accordance with an uneven surface of the unfixed toner image. Accordingly, the fixing roller touches a toner image surface a part by a part so as to cover the toner image surface a part by a part. Therefore, heat fixing can be preferably performed on an unfixed color toner image which contains more toner than an unfixed monochrome image.
Further, due to an effect of restoration from the deformation of the elastic layer at an exit of the fixing nip area, it becomes possible to improve releasability from color toner whose offset occurs more easily as compared with offset of monochrome toner.
Furthermore, a nip shape of the fixing nip area has an upward convexity (toward a fixing roller side) (so-called, a reverse nip shape). This makes it possible to improve paper stripping performance. As a result, without any stripping means such as a stripping claw, paper can be stripped (self-stripping). This consequently solves an image defect caused by the stripping means.
Further, in such a color fixing device, a nip width of the fixing nip area is required to be wider for increasing a process speed (a speed for carrying recording material). As methods for widening the nip width, there are two methods, i.e., a method in which a thickness of the elastic layer of the fixing roller is increased and a method in which a diameter of the fixing roller is increased.
However, in a case where a process speed is increased in a conventional configuration in which heating means is in a fixing roller, the method in which a thickness of an elastic layer is increased may result in insufficient heat supply to a surface of the fixing roller. This is because the elastic layer has a low heat conductivity. This may consequently lower a temperature of the surface of the fixing roller. Meanwhile, in the above case in the conventional configuration, the method in which a diameter of the fixing roller is increased may result in a larger heat capacity of each roller. This may lengthens a time for warming up or increases power consumption.
In order to solve such problems, in recent years, a color fixing device of a belt fixing system has been used increasingly. The belt fixing system is configured by stretching a fixing belt over a fixing roller and a heat roller and causing the fixing roller and the pressure roller to press against each other via the fixing belt.
This type of the fixing device of the belt fixing system has a short warming-up time because the fixing belt to be heated has a small heat capacity. Further, in this type of the fixing device, it is not necessary to provide a heat source such as a halogen lamp inside the fixing roller. This makes it possible to provide a thicker elastic layer which has a low hardness. For example, the elastic layer may be made of sponge rubber. As a result, a wide nip area can be ensured.
For further shortening the warming-up time in the fixing device of the heat roller fixing system or the belt fixing system as described above, (i) a heat capacity of a member to be heated (a fixing roller or a fixing belt) may be lowered or alternatively (ii) an output of heating means for heating the member to be heated may be increased. However, in a case where the heat capacity of the member to be heated is lowered or the output of the heating means is increased, an optimal temperature control becomes difficult. As a result, a temperature of a member to be heated varies to a large extent in the vicinity of a target temperature. This consequently causes a temperature change (temperature ripple) having a wide temperature range.
In a temperature control method of a conventional fixing device, temperature control is carried out on a temperature control target object such as the fixing member or the pressure member. In the temperature control, electricity to the heating means is turned on if the current temperature of the temperature control target object is lower than a target temperature; electricity to the heating means is turned off if the current temperature of the temperature control target object is higher than the target temperature. However, in the above temperature control, temperature rise may not stop immediately when the temperature of the temperature control target object reaches the target temperature and the temperature control target object may be overheated. Such a problem may occur in the following cases: (i) a case where the heat capacity of the temperature control target object is low; (ii) a case where an amount of heat supplied by the heating means is large; (iii) a case where a thermal time constant of the temperature control target object is small; or (iv) a case where a thermal time constant of temperature detection means is small which temperature detection means detects a temperature of the temperature control target object. Further, when heating by the heating means is stopped, a drastic temperature fall may occur due to an influence of heat dissipation or heat load.
In a case where a large temperature ripple occurs, toner may excessively melt and the melted toner may act as an adhesive between recording paper and the member to be heated (the fixing roller or the fixing belt). In such a case, the recording paper may wind around the heating member or, even if a problem does not occur to the extent that the recording paper winds around the heating member, an image surface after fixing may become coarse and image deterioration may occur. Meanwhile, overheating means supply of excessive thermal energy. Accordingly, overheating increases power consumption. In addition, a fixing error may also be caused by insufficiently melt toner due to an insufficient amount of heat supplied to the member to be heated.
In order to solve the above problems, Patent Literature 1 discloses PID control as a method for precisely controlling power supply to the heating means.
In this PID control, proportional control (P-operation), integral control (I-operation), derivative control (D-operation) are combined, and a current output to the heating means is determined based on a current temperature state, a past control state, and the like. If respective parameters of these three operations are well adjusted for a control target object, the temperature ripple can be drastically suppressed.
In Patent Literature 1, an output value of the heating means is calculated in detail based on temperature information from temperature detection means that detects a temperature of the member to be heated. In accordance with the calculated output value, an amount of power to be supplied is precisely controlled so that the temperature ripple is suppressed.
However, in the technique of Patent Literature 1, it is required to shorten a control cycle of power supply to the heating means, for suppressing the temperature ripple. Accordingly, a high-speed control system is required.
Further, in a case where the temperature of the member to be heated drastically changes in a short period of time, an output to the heating means is required to promptly increase to the maximum value for effectively suppressing a decrease in temperature and/or the temperature ripple. However, in the case where the PID control as disclosed in Patent Literature 1 is used, a delay may occur in an operation for suppressing the decrease in temperature by increasing the output to the heating means to the maximum. In such a case, the decrease in temperature may not be sufficiently suppressed. Further, in the configuration of Patent Literature 1, if the output to the heating means is promptly changed in a case where the temperature of the member to be heated drastically changes in a short period of time, the temperature control may become unsteady. As a result, the temperature ripple may not be sufficiently suppressed.
In addition, according to the technique of Patent Literature 1, for enhancing a temperature ripple suppressing effect provided by a control system, control is carried out in a very-short control cycle. Accordingly, a cycle of change in output becomes short. This may cause a higher harmonic noise in an AC supply. Further, in a case where the control cycle is shortened, each parameter cannot be easily fixed. Therefore, adjustment of the various parameters is often troublesome.
In order to solve the above problems, Patent Literatures 2 to 4 disclose a method for controlling the temperature ripple by a simpler control system. In this method, a duty ratio of a power waveform of power supply to the heating means is determined, based on (i) a temperature difference between the current temperature and a target temperature and (ii) a state of current temperature transition. Then, the power supply to the heating means is subjected to duty control.
More specifically, in Patent Literature 2, a table to be used in temperature control is chosen from among a plurality of tables, in accordance with a time taken for a rise of a temperature to a predetermined temperature at the time when the power supply is turned on. Then, by using the table selected, the duty control is performed.
In Patent Literature 3, a temperature division is determined based on a temperature difference between the current temperature and a target temperature. Then, in accordance with a change between the current temperature division and a previous temperature division, the duty control is performed.
In Patent Literature 4, according to a change in a state of feeding recording material, the power supply to the heating means is subjected to the duty control. Here, the duty ratio is set at 100% when the recording material passes. Meanwhile, the duty ratio is set based on a temperature difference and a temperature transition state in a period after a sheet of recording material has passed and before a next sheet of recording material is fed.
Citation List
[Patent Literature]
Patent Literature 1
Japanese Patent Application Publication, Tokukaihei, No. 9-258601 A (Publication Date: Oct. 3, 1997)
Patent Literature 2
Japanese Patent Application Publication, Tokukai, No. 2000-330418 A (Publication Date: Nov. 30, 2000)
Patent Literature 3
Japanese Patent Application Publication, Tokukai, No. 2007-3663 A (Publication Date: Jan. 11, 2007)
Patent Literature 4
Japanese Patent Application Publication, Tokukai, No. 2008-134377 A (Publication Date: Jun. 12, 2008)